A brushless direct-current (BLDC) motor may be classified into a core type (or a radial type) and a coreless type (or an axial type), each having a generally cup-shaped (cylindrical) structure, according to whether or not a stator core exists.
A BLDC motor of a core type structure may be classified into an inner magnet type including a cylindrical stator where coils are wound on a plurality of protrusions formed on the inner circumferential portion thereof in order to form an electronic magnet structure, and a rotor formed of a cylindrical permanent magnet, and an outer magnet type including a stator where coils are wound up and down on a plurality of protrusions formed on the outer circumferential portion thereof, and a rotor formed of a cylindrical permanent magnet on the outer portion of which multiple poles are magnetized.
In a conventional outer magnet type BLDC motor, a main path of a magnetic flux forms a magnetic circuit starting from a permanent magnet of a rotor and proceeding toward a stator via an air gap, and proceeding toward the permanent magnet again and in the direction of a yoke.
In a conventional inner magnet type BLDC motor, a plurality of T-shaped core portions on a stator core around which coils are wound, protrude inwards. Also, the inner side ends of the respective core portions form a circle of a predetermined diameter. Also, a rotor is mounted in an inner space of the inner magnet type BLDC motor in which a cylindrical permanent magnet including a rotational shaft is attached, or at the center of the inner magnet type BLDC motor in which a ring-shaped permanent magnet is attached to a cylindrical yoke including a rotational shaft. The inner magnet type BLDC motor rotates in the same manner as that of the outer magnet type BLDC motor.
The magnetic circuit in the above-described core type BLDC motor has a symmetrical structure in the radial direction around the rotational shaft. Accordingly, the core type BLDC motor has less axial vibration noise, and is appropriate for low-speed rotation. Also, since a portion occupied by an air gap with respect to the direction of the magnetic path is extremely small, a high magnetic flux density may be obtained even if a low performance magnet is used or the plurality of magnets is reduced. As a result, a big torque and a high efficiency may be obtained.
However, such a yoke structure may cause a material loss of a yoke when manufacturing a stator. In addition, a special expensive dedicated winding machine should be used in order to wind coil on the yoke due to a complex structure of the yoke when mass-producing. In addition, a mold investment cost is high at a time of manufacturing a stator, to thus cause a high facility investment cost, in the case of the core type BLDC motor.
Meanwhile, in order to overcome the shortcomings of the core type BLDC motor, the present applicant proposed a double rotor structure that offsets axial vibrations that occurs when the rotor rotates with each other, and increases torque more than twice, as a coreless type axial gap type BLDC motor through Korean Patent Registration No. 213571.
The above-described conventional double rotor BLDC motor, forms a magnetic circuit of a symmetrical structure with respect to the stator and the rotating shaft, to thereby increase an amount of stator coils twice as much as that of a single rotor BLDC motor, by using first and second rotors and a stator, and to also increase the plurality of field magnets twice as many as that of a single rotor BLDC motor, and to thus increase a driving current and magnetic flux density twice as many as those of a single rotor BLDC motor, and to thus obtain a torque at least two times as that of a single rotor BLDC motor of an identical axial gap type.
The axial gap type coreless motor has several advantages, but has a high magnetic resistance since a portion occupied by an armature winding is formed of an air gap and thus has a low magnetic flux density of the air gap compared to the amount of magnets used, to thereby cause the efficiency of the motor to be low.
In addition, if the plurality of turns of the armature winding is increased in order to implement a high torque motor, the air gap should be further increased, and thus the magnetic flux density should be decreased, to thereby result in a further reduction of the efficiency of the motor.
Thus, the axial gap type coreless motor has drawbacks that high-performance magnets should be used or the amount of magnets should be increased, to thus ultimately raise the price of the product. Furthermore, if the amount of wound coils is increased in order to improve the output of the motor, the air gap with respect to the rotor is increased proportionally because of an air-core structure, thereby increasing the magnetic resistance and thus reducing the efficiency of the motor.
In addition, in Korean Patent Laid-open Publication No. 10-2010-31688, was proposed an axial gap type rotating machine in which the amount of the magnetic flux is not reduced, magnets are not scattered even by the centrifugal force and by the thermal circulation driving, while the magnets can be firmly fixed to the yoke of the rotor.
To this end, in Korean Patent Laid-open Publication No. 10-2010-31688, was proposed a structure that a concave portion is formed on a surface of a rotating plate facing a stator and a permanent magnet gets stuck in the concave portion so as to have protrusions from the surface of the rotating plate. However, Korean Patent Laid-open Publication No. 10-2010-31688 discloses the same cureless motor as Korean Patent Registration No. 213571 does. Thus, if the amount of wound coils is increased in order to improve the output of the motor, the air gap with respect to the rotor is increased proportionally because of an air-core structure, thereby increasing the magnetic resistance and thus reducing the efficiency of the motor. In addition, in Korean Patent Laid-open Publication No. 10-2010-31688, was not proposed a structure that an opposite area with respect to a stator coil is optimized according to the use of a bar magnet.
To overcome these problems, the applicant has proposed an axial gap type core motor in Korean Patent Registration No. 213571.
However, the conventional axial gap type core motor has a structure that a plurality of split cores on which stator coils are wound are in the form of a square shape, and a plurality of magnets of a rotor opposing the plurality of split cores are in the form of a trapezoidal or square shape, in which the opposite areas of the rotor magnets facing the stator cores are not optimized. Moreover, since the split cores are configured into a rectangular shape, a mutual gap is wide and a fill factor of coils is low.
Moreover, the conventional axial gap type core motor has a structure that part of the split cores are inserted into respective bobbins on which coils are wound, the rest of the split cores are assembled on the other sides of the respective bobbins, and two parts of the split cores are adhered by a caulking process, to thereby complete the split cores. In this case, the fabricated armature windings are arranged and fixed on a printed circuit board (PCB), and then wired and injection molded, to thus manufacture a stator.
However, since the conventional axial gap type core motor has a structure that a plurality of split cores on which stator coils are wound are integrated by an insert injection molding process, and thus the insert injection molding process is added, working manpower is increased. Further, since the insert injection molding process corresponds to one of essential requirements, production costs have also increased. In addition, since a separate PCB should be used in order to mutually wire coils wound on the insulation bobbin, the working efficiency was reduced due to the complex and tedious manufacturing process.
Meanwhile, the stator core is usually formed by molding a large plurality of silicon steel plates of 0.35 to 0.5 mm thick into a predetermined shape, and then laminating the molded silicon steel plates. In the case of an integral core, a magnetic flux density in the air gap is not uneven due to the influence of slots into which coils are wound, to thus cause occurrence of a cogging torque phenomenon and torque ripples of non-uniform torque.
In order to reduce the cogging torque and torque ripples, a lot of slots are formed in the stator core, or secondary salient poles or secondary slots are slot formed therein, or the stator core is configured to employ a skew structure.
Also, a radial gap type motor is designed to have centers of T-shaped portions that are close to magnets and leading ends of the T-shaped portions that are distant away from both ends. However, the radial gap type motor has a core in which a large plurality of silicon steel plates of an identical shape are laminated, and thus the core can be rounded only in the axial direction.
Moreover, to reduce noise due to the torque ripples, the magnet is skew magnetized, or both sides of a segment magnet are edge processed (shaped), to thereby make distribution of lines of magnetic force have shape of a sinusoidal wave.
However, the above-mentioned solutions have problems that a core is rounded only in the axial direction, it is difficult to perform coil windings or bobbin molding, or costs for magnetization are expensive.